The belt-type CVT of a vehicle includes a steel belt configured by attaching a plurality of a platelike component of steel (elements) to a continuous circular steel ring side by side, and a pair of pulleys having a variable groove width. In addition, the steel belt is wound between the pair of pulleys in an endless annular, and power transmission is performed from one pulley to the other pulley through the steel belt. The respective elements are disposed by being sandwiched between two bundles of steel rings. Power from an engine is input to one pulley, is transmitted to the other pulley through the steel belt, and is output. At that time, the effective diameter of each of the pulleys is made to vary by changing the groove width of each of the pulleys, and thus continuous gear change occurs.
Elements for the belt-type CVT are shaped into a product shape by cold-punching the steel sheet. Therefore, it is necessary for a material suitable for the elements to have high hardness, high wear resistance, and cold punching properties. As a material satisfying these demands, Patent Document 1 and Patent Document 2 suggest the following steel.
Patent Document 1 discloses steel which includes, by mass %, C: 0.1% to 0.7%, Cr: 0.1% to 2.0% and S: 0.030% or less, and which is subjected to a carburizing treatment (carburizing and quenching—tempering) after the punching. The steel is a low and medium carbon steel that is soft and thus the lifetime of a precision mold used for punching increases. As a result, the machining costs may be reduced. In addition, the steel secures the hardness necessary for a surface layer (a depth of 50 μm from a surface) by the carburizing treatment. Furthermore, the steel is lows and medium carbon steel, and thus toughness of a core of a carburized product may be maintained to be high. As a result, an impact value of the carburized product itself may be improved.
Patent Document 2 discloses high carbon steel which includes, by mass %, C: 0.70% to 1.20% and in which the particle size of carbides dispersed in a ferrite matrix is controlled. The steel has improved notch tensile elongation having a close relationship with punching workability, and thus the punching workability thereof is excellent. In addition, the steel further includes Ca, and thus morphology of MnS is controlled. As a result, the punching workability is further improved.